The invention relates to a method, a switching device, a telecommunication system and a terminal station, in particular for a GSM-based General Packet Radio Service system (GPRS), that allow a subscriber station to select a predetermined network of several packet data networks (PDNs) connected to a gateway GPRS support node (GGSN). The packet data networks can be any kind of packet data networks or Internet Service Providers (ISPs).
The standardization of the GSM General Packet Radio Service (GPRS) is currently in progress at the European Telecommunication Standards Institute (ETSI). GPRS is a new GSM-service that provides actual packet radio access for mobile GSM users. According to the GPRS system radio resources are reserved only when there is something to send (due to the packet nature of this system) and the same radio resource is shared by all mobile stations in a cell, providing effective use of the scarce resources. GPRS facilitates a variety of applications, such as telemetry, train control systems, interactive data access, charging systems and Internet browsing using WorldWideWeb.
Contrary to the circuit switched nature of the GSM network, the operation of GPRS is adapted to offer a connection to a standard data network (using protocols such as TCP/IP, X.25 and CLNP). By contrast, the conventional GSM network was originally designed to offer only circuit switched voice sessions. The packet-orientated GPRS network infrastructure introduces new functional elements which will hereinafter be briefly described with reference to FIG. 1.
It should be noted that still some cooperation exists between elements of the current GSM services and the new GPRS network. On the physical layer, resources can be refused and some common signaling features exist. In the same radio carrier there can be time slots reserved simultaneously for circuit-switched and GPRS use. The most optimum resource utilization is obtained through dynamic sharing between circuit-switched and GPRS channels. During the establishment of a circuit switched call, there is still enough time to pre-empt the GPRS sources for circuit-switched cells that have higher priority.
Interaction of GSM Network and GPRS Network
FIG. 1 is a simple overview of the interaction of GSM circuit switched features and elements of the GPRS packet switched system. The GPRS Support Node GSN is the main element and provides connection and interworking with various data networks, mobility management by means of the GPRS registers and of course the delivery of data packets to mobile stations GPRS-MS independently of their location. Physically, the GSN can be integrated in the mobile switching center MSC of the PLMN (Public Land Mobile Network). Alternatively it can be a separate network based on the architecture of data network routers. The user data flows between the GSN and the base station sub-system (BSS) and a signaling is exchanged between the MSC and the GSN.
Thus, the GPRS provides a bearer service from the boundary of a data network to a GPRS MS. The users of the bearer service are the public network layer software packages (such as IP, OSI CLNP and X.25). Also, GPRS-specific applications will use the GPRS service.
GPRS uses a packet mode technique to transfer high-speed and low-speed data and signaling in an efficient manner. GPRS optimizes the use of the network resources and minimizes the load on the radios system. Strict separation between the radio subsystem and network subsystem is maintained allowing the network subsystem to be reused with other radio access technologies. GPRS as such does not mandate changes to an installed MSC base.
New GPRS radio channels are defined and the allocation of these channels is flexible: From 1 to 8 radio interface time slots can be allocated per TDMA frame and time slots are shared by the active users with the up-link and down-link allocated separately. The radio interface sources can be shared dynamically between speech and data services as a function of service load and operator preference. Various radio channel coding schemes are specified to allow bit rates from 9 to more than 150 Kbyte/s per user. It is even estimated that a raw data rate of up to 200 Kbyte/s can be obtained per user.
As explained above, applications based on standard data protocols are supported and interworking is defined with IP networks and X.25 networks. Specific point-to-point and point-to-multipoint services are supported for applications such as traffic telemetric and UIC train control. GPRS also allows a short message service (SMS) transfer over the GPRS radio channels.
GPRS is designed to support from intermittent and bursty data transfers through to occasional transmission of large volumes of data. Four different Quality of Service (QoS) levels (there QoS are set initially during a PDP-context activation procedure as explained below) are supported. GPRS is designed for fast reservation to begin a transmission of packets, to 0.5 to 1 seconds. Charging will typically be based on the amount of data transferred due to the packet nature of transmission.
Terminal Stations Supporting GPRS
In GPRS three different classes of GPRS mobile stations are supported: a class-A MS can operate GPRS and other GSM services simultaneously. A class-B MS can monitor control channels for GPRS and other GSM services simultaneously, but can only operate one set of services at one time. A class-C GPRS MS can exclusively operate GPRS services.
Data Packet Transmission
Having generally set up the GPRS support nodes GSN in FIG. 1, of course one of the main problems in GPRS network is the routing of data packets to/from a mobile station MS. This problem can be divided into two sub-problems, namely the data packet routing and the mobility management.
Data packet routing to a mobile station MS is a problem in the GPRS network, since the mobile station""s data network address typically has a static routing mechanism, while the mobile station MS can roam from one network to another. One approach for a data packet routing in a mobile environment is the concept of mobile IP. (C. Perkins (editor): xe2x80x9cIP Mobility Support, draft ietf-mobileip-protocol-11.txtxe2x80x9d, July 1995, Work in progress in the Internet Engineering Task Force).
Mobile IP enables the routing of IP datagrams to mobile hosts, independent of the sub-network of point of attachment. Another approach is taken in the system for cellular digital packet data (CDPD) where the routing to mobile host is handled internally by the network (CDPD Industry Input Coordinator, xe2x80x9cCellular Digital Packet Data System Specificationxe2x80x9d, Release 1.0, July 1993).
The standard mobile IP concept does not fit exactly in the GPRS environment because of the requirement that network protocols other than IP must also be supported. Therefore, for the routing of the data packets the structure of the telecommunication network in FIG. 1 (comprising general GPRS nodes GSN) is constructed in a concept similar to the mobile IP concept as is shown in FIG. 2.
GPRS Support Nodes
In FIG. 2, GPRS introduces two new network nodes in the GSM PLMN: The serving GPRS support node (SGSN), which is at the same hierarchical level as the MSC (Mobile Switching Center) keeps track of the individual mobile stations"" location and performs security functions and access control. The SGSN is connected to the base station system with frame relay. Thus, the main functions of the SGSN are to detect new GPRS MSs in its service area, to handle a process of registering the new MSs in the GPRS registers, to sent/receive data packets to/from the GPRS MS and keep a record of the location of MSs inside of its service area. The subscription information is stored in a GPRS register where the mapping between a mobile station""s identity (such as MS-ISDN or IMSI: International Mobile Station Identity) and the PSPDN address is stored. The GPRS register acts as a data base from which the SGSNs can ask whether a new MS in its area is allowed to joint the GPRS network.
The gateway GSN (GGSN) provides interworking with external packet switched networks and is connected with SGSNs via an IP based GPRS backbone network (IP: Internet protocol). The aforementioned GPRS registers can be provided in the HLR which is thus enhanced with the GPRS subscriber information. Optionally, the MSC/VLR can be enhanced for more efficient coordination of GPRS and non-GPRS services and functionality: e.g. paging for circuit switched calls which can be performed more efficiently via the SGSN and combined GPRS and non-GPRS location updates.
As also shown in FIG. 2 (although not relevant in the present application), the SGSN of course cooperates with a short message service gateway MSC SMS-GMC via a Short Message Service interworking MSC (SMS-IWMSC).
Furthermore, it should be noted that the SGSN performs authentication and cipher setting features based on the same algorithms, keys and criteria as in existing GSM. GPRS uses a ciphering algorithm optimized for packet data transmission.
GPRS Access by a Mobile Station
In order to access the GPRS services, the mobile station must first make its presence known to the network by performing a GPRS attach. This operation establishes a logical link between the mobile station and the SGSN and makes the mobile station available for SMS over GPRS, paging via SGSN and notification of incoming GPRS data. In order to send and receive GPRS data, the mobile station must activate the packet data address (PDN-address) that it wants to use. This operation makes the mobile station known in the corresponding GGSN and interworking with external data networks can commence.
User data is transferred transparently between the mobile station and the external data networks with a procedure known as encapsulation and tunneling (the exchange of tunneling messages is part of the PDP-context activation procedure): data packets are equipped with GPRS-specific protocol information and are transferred between the mobile station and the GGSN. This transparent transfer method lessens the requirement for the GPRS PLMN to interpret internal data protocols and it enables easy introduction of additional interworking protocols in the future. User data can be compressed and detected with retransmission protocols for efficiency and reliability.
Thus the GPRS support node in its general form (GSN) contains functionality required to support GPRS. In one PLMN, there may be more than one GSN as is seen in FIG. 3.
The gateway GPRS support node (GGSN) is the node which is accessed by the packed data network due to evaluation of the so-called PDP address. This address contains routing information for attached GPRS users. The routing information is used to tunnel protocol data units (PDUs) to the current point of attachment of the mobile station, i.e. to the respective serving GPRS support node (SGSN). The GGSN may request location information from the HLR via the optional Gc interface. The GGSN is the first point of PDN (Packet Data Network) interconnection with a GSM PLMN, supporting GPRS (i.e. the Gi reference point is supported by the GGSN).
Intranetworks and Internetworks Connected to GPRS
While FIG. 1 shows the general structure of the embedding of the GPRS functionalities in a GSM system, FIG. 3 shows additional networks within the PLMNs needed as GPRS backbone networks.
The intra-PLMN backbone network is the internet protocol network interconnecting GSNs within the same PLMN. The Inter-PLMN backbone network is the IP network interconnecting GSNs and intra-PLMN backbone networks in different PLMNs. Every intra-PLMN backbone network is a private IP network intended for GPRS data and GPRS signaling only. Such a private IP network is an IP network to which some access control mechanism is applied in order to achieve a required level of security.
Intra-PLMN backbone networks are connected via the Gp interface using border gateways (BGs) and an inter-PLMN backbone network. The Inter-PLMN backbone network is selected by a roaming agreement that includes the BG security functionality. The BG is not defined within the scope of GPRS. The inter-PLMN backbone can be a packet data network. For example, the intra-PLMN backbone network can be a corporate network and the packet data network can be a public internet or a leased line.
Finally, the HLR shown in FIG. 2 contains the GPRS subscription data and routing information. This HLR is accessible from the SGSN via the Gr interface and for roaming mobile stations MSs HLR may be in a different PLMN than the current SGSN to which the mobile station is connected.
Therefore, in FIG. 3 the HLR can be located in PLMN A or PLMN B.
Example of GPRS Communication
Having described the general architecture of the GPRS system in FIGS. 1-3, FIG. 4 shows an illustrative example how the routing of information can be performed in such a system. As shown in FIG. 4, within the GPRS mobile communication system there are 3 different routing schemes and thus 3 examples of possible applications for the present invention are as follows:
mobile originated message (path P1)
mobile terminated message when the mobile station (MS) is in its home network (path P2); and
mobile terminated message when the mobile station (MS) has roamed to a network of another GPRS operator (path P3).
As in FIG. 3, also in FIG. 4 the operator""s GPRs network consists of multiple GSNs and an intra-operator backbone network. The intra-operator backbone network connects the support nodes of one operator using operator-specific network protocols that can be different for each operator. With interworking capabilities, the GGSN can, however, be connected to data networks and to an inter-operator backbone network that connects the GPRS networks of different operators using one standard protocol.
The main benefits of this proposed architecture are its flexibility, scalableness and interoperability. This approach allows each operator PLMN A, B to implement an individual backbone network using any protocol, by communications while other GPRS operators are implemented using only one common protocol. ETSI has selected IPv6 to be the main backbone protocol in the future. IPv4 has been selected as the intermediate backbone protocol.
As is seen in FIG. 4, from standpoint of data network, the GPRS network resembles a sub-network in the data network. For example, in the internet, the GGSN acts like an IP router behind which the entire GPRS network is hidden. A computer in the internet network then sees the GPRS as an IP sub network to which the messages are sent as if the GPRS network was a completely standard internet implementation. The routing mechanism in the data network is then exactly the same as with the normal internet receiver case.
According to a first example of data routing shown in FIG. 4 and being related to path P1, the GPRS mobile station sends a data packet, i.e. a packet data unit PDU of a public switched packet data network PSPDN to a data network. The PSPDN PDU data packet is sent using the LLC (Logical Link Control) protocol over the air interface to the GPRS Serving Support Node SGSN currently serving the GPRS mobile station MS. In case the GPRS Serving Support Node SGSN has received the data packet error free, it encapsulates the PSPDN PDU data packet into the GPRS backbone network data packet that is sent to the GPRS Gateway Support Node (GGSN) handling the traffic from the GPRS mobile station MS to the data networks. The GPRS gateway support nodes GGSN decapsulates the PSPDN PDU data packet and forwards it to the appropriate data network.
As shown in FIG. 4, a second example for the application of the invention is related to a path P2 where a host in a data network is sending a PSPDN PDU data packet to a GPRS mobile station MS located in the home GPRS network. Here, compared to the first example outlined above, the PSPDN PDU data packet is routed in reverse direction using the routing mechanisms in the data network until the PSPDN PDU data packet arrives at the GPRS Gateway Support Node GGSN. In the GPRS Gateway Support Node the PSPDN address of the GPRS mobile station MS is extracted and the current location of the GPRS mobile station MS is mapped. Then, routing of the PSPDN PDU data packet in the home GPRS network is carried out. Thus, the PSPDN PDU data packet is first encapsulated into a backbone network and then sent to the GPRS serving support node SGSN currently serving the GPRS mobile station MS.
The last example shown in FIG. 4 relates to path P3 and is almost identical to example P2. Here, the GPRS mobile station MS has roamed to another GPRS network and the home GPRS network must send the PSPDN PDU data packet over the inter-operator backbone network to the visited GPRS network. Thus, according to this example, there is involved an additional GPRS Gateway Support Node GGSN to provide the data packet to the roaming GPRS mobile station MS. Then, the visited GPRS network routes the PSPDN PDU data packet further to the appropriate GPRS Serving Support Node, as is outlined above with respect to the second example.
Log-on Procedure of GPRS-MS
A typical log-on procedure of a GPRS mobile station MS which desires the transmission of data packets is shown in FIG. 5. The main objective of this log-on procedure is to send the PSPDN address of the GPRS mobile station MS to the GPRS network, to report on the current whereabouts of the GPRS mobile station MS, to create entries for the assigned PSPDN address in the routing table of the GPRS gateway support node GGSN and to initiate charging in statistical procedures, respectively.
During the GPRS log-on procedure, the context (the content or the parameter sets) of the logical link between the MS and the SGSN is established using the GSM stand-alone dedicated control channel (SDCCA) as a carrier. During the context establishment, the GPRS mobile station is also authenticated and ciphering parameters are exchanged between the GPRS mobile station MS and the GPRS Serving Support Node SGSN (this authentication/ciphering procedure is carried out separately to the PDP context activation described below; see the GSM 03.60 document).
The registration is then forwarded to the GPRS Gateway Support Node in which the location of the GPRS mobile station MS is updated. Here, the GPRS Gateway Support Node GGSN may inform a previous GPRS Serving Support Node SGSN to remove the GPRS mobile station MS from the previous registers. In case the GPRS log-on procedure is successful, the GPRS mobile station enters the stand-by state. Finally, the GPRS mobile station can exit the GPRS service by initiating a GPRS log-off procedure similar to the log-on procedure.
PDP-context Activation Procedure
At PDP context activation, the SGSN establishes a so-called PDP context to be used for routing purposes inside the GPRS PLMN with the GGSN that the GPRS subscriber is using. Such a PDP context activation procedure is shown in FIG. 6.
A point-to-point (PTP) GPRS subscription contains the subscription of one or more PDP addresses (e.g. in the HLR) Each PDP address is described by an individual PDP context in the mobile station MS, the SGSN and the GGSN. Every PDP context exists independently in one of two PDP states. The PDP state indicates whether the PDP address is activated for data transfer or not. All PDP contexts of a subscriber are associated with the same MM context for the IMSI of that subscriber.
Thus, the PDP context is an information set held in the mobile station MS and GSNs for the PDP address as is described in xe2x80x9cDigital Cellular Telecommunication System (Phase 2+); General Packet Radio Services (GPRS); GPRS Tunneling Protocol (GTP) across the Gn and Gp interface; (GSM 09.60 proposed version 1.1.0), Draft TS100 960 proposed V1.1.0 (published by the European Telecommunications Standards Institute ETSI, June 1997).
Upon receiving an activate PDP context request message, the SGSN shall initiate procedures to set up PDP contexts. Therefore, a valid request initiates the creation of a tunnel between a PDP context in a SGSN and a PDP context in a GGSN. That is, after a successful PDP context activation procedure during or after the log-on procedure in FIG. 5, a PDP context has been agreed upon between the SGSN and the GGSN (and thus the GPRS mobile station), which will be used for the packet data transmission. The list of PDP context information parameters is shown in table 5 of the GSM 0360 proposed version 2.0.0 document (published by ETSI, May 1997).
The conventional PDP context activation procedure in FIG. 6 comprises the following four steps S1, S2, S3, S4.
In step S1, the mobile station MS sends an activate PDP context request (TLLI, QoS requested, NSAPI) message to the SGSN. The mobile station MS indicates that it wishes to use a dynamic PDP address by selecting a NSAPI (network layer service access point identifier) referring to a PDP context that indicates a dynamic address of the desired type.
In step S2, security functions are executed.
In step S3, the SGSN checks that the NSAPI matches a PDP context in the subscription data which were stored in the SGSN during the GPRS log-on (attach). If the mobile station MS requests a PDP context with dynamic address, then the SGSN lets a GGSN allocate the dynamic address (the GGSN used is either the GGSN address stored in the PDP context or, if this field is empty, a suitable GGSN chosen by the SGSN). The SGSN may restrict the required QoS values given its capabilities, the current load and the subscribed QoS level.
Thus, in step S3xe2x80x2, the SGSN sends a create PDP context request (IMSI, PDP type, PDP address, QoS negotiated, TID) message to the affected GGSN. The PDP address is set to zero if the dynamic address is requested. The GGSN creates a new entry in its PDP context table. The new entry allows the GGSN to route PDP PDUs between the SGSN and the external PDP network.
In step S3xe2x80x3, the GGSN then returns a Create PDP context response (TID, PDP address, BB protocol, Cause) message to the SGSN. The PDP address is included if the GGSN allocated a PDP address. The BB protocol indicates whether TCP or UDP shall be used to transport user data on the backbone network between the SGSN and GGSN. The create PDP context messages are send over the GPRS backbone network.
In step S4, the SGSN inserts the PDP address received from the GGSN in its PDP context. The SGSN returns an Activate PDP Context Accept (TLLI, PDP type, PDP address, NSAPI, QoS negotiated, Cause) message to the MS. After step S4, the SGSN is now able to route PDP PDUs between the GGSNs and the mobile station MS.
For each PDP address, a different quality of service (QoS) may be requested. For example, some PDP addresses may be associated with e-mail that can tolerate lengthy response times. Other applications cannot tolerate delay and demand a very high level of throughput, interactive applications being one example. These different requirements are reflected in the QoS parameter. The QoS values are defined in GSM 02.60. If a QoS requirement is beyond the capabilities of a PLMN, the PLMN negotiates the QoS as close as possible to the requested QoS. The MS either accepts the negotiated QoS, or deactivates the PDP context. After a SGSN has successfully updated the GGSN, the PDP contexts associated with an MS is distributed as shown in subclause xe2x80x9cInformation Storagexe2x80x9d of the GSM 03.60.
If the PDP context activation procedure fails or if the Activate PDP Context Accept Cause parameter indicates a reject, then the MS may attempt another activation to the same PDP address up to a maximum number of attempts.
Whilst every GPRS mobile station must always carry out the procedure in FIG. 6., further details of the modified PDP context activation procedures can be taken from the aforementioned two ETSI documents (which also give a description of other abbreviations used for parameters in the above description generally known to the person skilled in mobile communications).
As explained above, in order to allow a packet data transmission from a GPRS mobile station MS to a packet data network supporting a packet data protocol like IP or X.25 (which is connected to the GGSN) in FIGS. 1 to 4, it is necessary that a log-on procedure or PDP context activation procedure is carried out as described with reference to FIGS. 5, 6. This activation procedure is used to create a tunnel between a PDP context in a SGSN and a PDP context in a GGSN.
Essentially, the PDP context can be seen as a set of parameters agreed upon between the SGSN and the GGSN for a packet transmission using a specific protocol. Typical parameters which have conventionally been used in this set of parameters are the MS-ID, the QoS, the NSAPI, the TEPI, and the PDP-address. In particular, a GPRS subscriber identified by an IMSI, shall once have one or more network layer addresses, i.e. PDP addresses, temporarily and/or permanently associated with it that conform to the standard addressing scheme of the respective network layer service used, e.g.:
an IP version 4 address;
an IP version 6 address; or
an X.121 address.
PDP addresses are activated and deactivated through MM procedures described in subclause xe2x80x9cPDP Context Activation and Deactivation Functionsxe2x80x9d in the GSM 03.60 document.
Once the tunnel has been set up by the PDP context activation, a packet data transmission can take place as explained for the examples 1, 2, 3 in FIG. 4. It should also be understood that the above set-up procedures need to be carried out in any telecommunication system that uses an embedded packet radio service within a conventional circuit switched PLMN environment.
As is seen in FIG. 7 (together with FIGS. 2, 3), there is a need to connect a large number of internet service providers ISP to a GPRS network (i.e. to the GGSN thereof) in order to attract as many customers as possible. In FIG. 7, even an intra-PLNM backbone network connected to a GPRS network (or a GGSN node thereof as is seen in FIG. 3) is considered as an internet service provider ISP, because technically there is no difference because in terms of interconnection both are connected to the GGSN.
As explained above, on the basis of the PDP context activation procedure, the current GPRS standard (GSM 03.60) already allows the possibility to interconnect the GGSN node to a large number of internal networks (ISPs). A subscriber can have a subscription (typically in the HLR) to one or several of such networks, e.g. subscription to his company internet (in FIG. 7: corporate network like ERINET at Ericsson) or to a packet data network (in FIG. 7: X.25 PDN) and to one or more internet service providers (in FIG. 7: local ISP, ISP1, ISP2). During the log-on and PDP context activation procedure, the SGSN will negotiate with the GGSN the PDP context for a particular network. However, at service activation, the subscriber station (i.e. the mobile station) does not have the possibility to flexibly indicate to the GPRS network which of his subscribed IPSs he would like to connect his session to.
Therefore, the object of the present invention is
to provide a method, a switching device, a telecommunication system and a terminal station, which allow a GPRS subscriber a more flexible use of several external networks connected to the GPRS.
This object is solved by a method for data communications between a first terminal station of a mobile radio telecommunication network and a second terminal station of a packet data communication network, comprising the following steps:
a) sending a network indication parameter indicating a predetermined packet data communication network from said first terminal station to a switching device of said mobile radio telecommunication network to which a plurality of packet data communication networks are connected;
b) selecting an access means in said switching device providing an access to the packet data communication network indicated by said network indication parameter; and
c) activating said selected access means to access a switching device of said indicated packet data communication network.
This object is further solved by a switching device for providing data communications between a first terminal station of a mobile radio telecommunication network and a second terminal station of one of a plurality of packet data communication networks connected thereto, comprising:
a) a reception means for receiving a network indication parameter indicating a predetermined packet data communication network from said first terminal station;
b) a plurality of access means each providing an access to one of said connected packet data communication networks;
c) a selection means for selecting an access means in accordance with said received network indication parameter; and
d) a control means for activating said selected access means to access a switching device of said indicated packet data communication network.
This object is also solved by a telecommunication system for providing packet data communications between a first and a second terminal station thereof, comprising:
a) at least one mobile radio communication network to which said first terminal station is connected; and
b) a plurality of packet data communication networks, said second terminal station being connected to one of said packet data communication networks; and
c) said communication networks being connected to a switching device which comprises:
c1) a reception means for receiving a network indication parameter indicating a predetermined packet data communication network from said first terminal station via said mobile radio communication network;
c2) a plurality of access means each providing an access respectively to one of said connected packet data communication networks;
c3) a selection means for selecting an access means in accordance with said received network indication parameter; and
c4) a control means for activating said selected access means to access a switching device of said indicated packet data communication network.
This object is further solved by a terminal station of a mobile radio telecommunication network for packet data communications to a predetermined terminal station of a packet data communication network, comprising:
a) a network indication parameter memory means for storing a plurality of network indication parameters respectively corresponding to a packet data communication network connected to said mobile radio telecommunication network through a switching device;
b) a selection means for selecting a network indication parameter from said memory means indicating a packet data communication network to/from which said terminal station is to transmit/receive packet data; and
c) a network request means for sending said selected network indication parameter to said switching device for requesting a connection to the packet data communication system indicated by said network indication parameter.
According to the invention, a network indication parameter is transferred to the SGSN which indicates the desired network, preferably during the PDP context activation procedure. The network indication parameter can be of a PDP type negotiated for the PDP context in the PDP context activation procedure. Thus, whilst the GPRS subscriber station was conventionally restricted to rely upon the SGSN to negotiate the appropriate network, according to the invention, any desired network can be prespecified during the PDP context activation or log-on procedure.
Further advantageous embodiments and improvements of the invention can be taken from the dependent claims. Hereinafter, the embodiments of the invention will be described with reference to the attached drawings. In the drawings, the same or similar reference numerals designate the same or similar elements or steps.